ABSTRACT
I. INTRODUCTION Water must be considered one of the most important factors in determining the permanence of aluminum adhesive-bonded joints in weathering exposure service conditions. In Chapter 7 on mechanisms of joint failure, we have primarily considered the response of the bulk adhesive or the adhesive in the interfacial area to physical stressing situations. No allowance was generally made in such cases for the fact that the adhesive joint, as originally fabricated, has also to exist over long time periods in the presence of the degrading chemical factors of the environment. Tetelman and McEvily (2863) have reported that basic continuum fracture mechanics only requires a small crack before a substantially decreased failure time or diminished joint strength should be observed. However, this failure can indeed become much more catastrophic and can be actuated by a lower stress loading when water is also present in the environment. Althof (2864), Wake (2865), and Comyn (2866) have contributed recent discussions concerning the influence of water on the stability and durability of bonded joints, which can be of special interest to the reader. These investigators have focused on the fact that water can attack the adhesive (both in the bulk phase and in the interphase region) as well as the adherend directly. Since most adhesives have polar sites in their chemical structure, it is natural that highly polar water can literally be attracted into their bulk and permeate toward the interfacial area. Such water entry can cause physical swelling and deformation, as will be discussed in more detail later. At the interface or interphase area, water can actually act to displace some of the bonding already in place between the adhesive and the adherend. The excellent resistance to water debonding of phenolic-modified structural adhesives in aluminum joints was early reported by DeLollis and Montoya (2867), Bolger (2868), Buck and Hockney (2869), and has been explained by the strong covalent bonding that can arise between the oxide and phenolic configurations resisting rupture by water. Finally, water can directly chemically attack the aluminum via a corrosion mechanism and undermine the bondline structure. In this regard, the method of aluminum surface pretreatment can be most important as established in the author's investigations (532-534,774).
